Many manufacturing processes involve the electroplating of copper on various surfaces including metal and conducting surfaces. Such copper platings are used to prevent corrosion, to increase electrical conductivity or thermal conductivity, and to serve as an adherent layer for additional metallic layers. Recently, such activity with regard to copper electroplating has been associated with the production of various electronic circuits and devices including circuit boards, integrated circuits, electrical contact surfaces, etc.
Traditional electroplating processes for copper involve the use of consumable counterelectrodes (or anodes). Here, the counterelectrode is made of metallic copper which is oxidized to soluble copper ions during the electroplating process. The counterelectrodes serve not only as the anode in the electroplating process but also as a source of copper ions in the electroplating bath to replace the copper ions consumed in the electroplating process at the cathode.
The copper electroplating process described in the instant application involves use of nonconsumable counterelectrodes or anodes. Such processes are advantageous because of potentially higher plating rates, better control of bath chemistry, smaller size of the plating apparatus and use of various copper salts (such as copper oxide) as sources of copper in the electroplating process. These copper salts are often readily available from etching procedures in the manufacture of various items including electronic circuit boards.
There are other advantages in the use of nonconsumable electrodes in copper electroplating. For example, geometry remains very constant so that there is better control of plating characteristics (plating rate, thickness, etc.) than in the case of consumable electrodes where electrode size and shape are constantly changing. Also, the copper electroplating process need not be interrupted to change electrodes if nonconsumable electrodes are used.
A particular difficulty in all copper electroplating is obtaining copper films of sufficient quality for a particular application. For example, in electrical applications, it is desirable to have ductile, smooth bright copper films with high electrical conductivity. Smooth films of uniform thickness are also highly desirable.
These desirable qualities of copper made by electroplating are usually obtained by the use of additives to the copper electroplating bath. The use of such additives improves brightness, improves the physical properties of the copper plating such as ductility, and increases micro-throwing power of the bath. Although this procedure often works quite well, it is under certain circumstances desirable to simplify the composition of the bath. For example, additives might decompose rapidly or use of additives might be inconvenient under certain circumstances.
Periodic reversal of current in the copper electroplating process is also known to improve the quality of copper platings and often permits more rapid electroplating of copper without degradation of plating quality (see the following references: C. C. Wan et al, Plating, 61, 559 (1974); J. Mann, Transactions of the Institute of Metal Finishing, 56, (2) 70 (1978), and H. L. Pinkerton et al, Plating, 59, 672 (1972)). Although periodic reversal of current works well with copper consumable anodes, with some types of nonconsumable anodes the reverse current tends to limit the lifetime of the anode.